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November 2000 1 MIC4043
MIC4043 Micrel
MIC4043
Low-Voltage Secondary-Side Shunt Regulator
Final Information
General Description
The MIC4043 is a shunt regulator optimized for secondary-
side regulation in low-voltage power supplies. Featuring an
output stage guaranteed to swing within 400mV of ground,
the MIC4043 can be used in power supplies operating down
to 1.8V, even with optoisolators requiring greater than 1.2V of
headroom.
In power supply applications, the MIC4043 normally drives
the LED of an optically isolated feedback circuit. The MIC4043
monitors a resistively-divided output voltage and sinks error
current through the optoisolator’s LED (secondary side); the
optoisolator’s transistor (primary side) provides this signal to
the controller’s feedback input. The MIC4043 is also practical
for other voltage-monitoring applications requiring an open-
collector output.
The MIC4043 replaces conventional ’431-type shunt regula-
tors to allow low-voltage applications where there is inad-
equate headroom for a 2.5V regulator in series with an
optoisolator. Replacing ’431-type devices requires only a
minor change to the way that the resistive-divider values are
calculated.
Typical Application
V
OUT
MIC4043
MIC38HC43BN
V
DD
V
REF
GND
COMP
V
OUT
R
T
/C
T
FB
I
SNS
2
1
4
3
8
5
7
6
Return
1
2
7
6
4
3
FBGND
SNKIN
V
IN
OPTICAL
ISOLATION
COMPENSATION
MIC4043
Low-Side Feedback
Control
PRIMARY SIDE
SECONDARY SIDE
R1
R2
V 1.245V R2
R1 1
OUT =+
200kHz DC-DC Flyback Converter
Features
•Ideal for 1.8V switching converters
•Low-voltage operation
400mV maximum saturation over
operating temperature range
•Easy to use
voltage in, current out
•2% voltage tolerance over operating temperature range
Applications
•Optically isolated low-volage power supplies
•Low-voltage discrete regulator control
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
SOT-143
M‘CAOASYMA R81 D 1.245V 1% Open Collector —40"C Io +85“C SOT-143
/
III III
MIC4043 Micrel
MIC4043 2 November 2000
Pin Description
Pin Number Pin Name Pin Function
1 IN Input: Supply voltage input.
2 SNK Sink (Output): NPN open collector output.
3 GND Ground
4 FB Feedback (Input): Feedback input from external voltage-divider network.
Pin Configuration
34
12
FB GND
SNK
Part
Identification
IN
RBxx
MIC4043
Ordering Information
Part Number Marking Voltage Tolerance Configuration Temperature Range Package Lead-Finish
MIC4043BM4 RB1D 1.245V 1% Open Collector –40°C to +85°C SOT-143 Leaded
MIC4043YM4 RB1D 1.245V 1% Open Collector –40°C to +85°C SOT-143 Pb-Free
Absolute Maximum Ratings (Note 1)
Input Voltage (VIN) ......................................................+15V
Output Voltage (VSNK) ................................................+15V
Storage Temperature (TS) ....................... –65°C to +150°C
ESD Rating, Note 3
human body model.................................................... 2kV
machine model ........................................................200V
Operating Ratings (Note 2)
Input Voltage (VIN) ......................................................+10V
Output Voltage (VSNK) ................................................+10V
Maximum Output Current (ISNK) ................................15mA
Temperature Range (TA) ........................... –40°C to +85°C
”u? _
_‘ II
November 2000 3 MIC4043
MIC4043 Micrel
Electrical Characteristics
TA = 25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted
Parameter Condition Min Typ Max Units
Reference Voltage, Note 4 1.245 V
Reference Voltage Tolerance ±1%
±2%
Supply Current ISNK = 0mA 35 65 µA
70 µA
Transconductance 1mA < ISNK < 15mA 3.5 150 S
∆ISINK/∆VIN 2S
Output Transistor ISNK = 15mA 160 250 mV
Saturation Voltage 400 mV
Output Leakage VSNK = 5V, output transistor off 0.5 µA
1µA
Note 1. Exceeding the absolute maximum rating may damage the device.
Note 2. The device is not guaranteed to function outside its operating rating.
Note 3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. Machine model, 200pF.
Note 4. Reference voltage is not referenced to ground. The reference is between pins IN and FB.
Test Circuits
IN
R
A
1k R1
R2
33k
C1*
50Ω
R3
GND FB
SNKOUT
RETURN
MIC4043
Floating
Bench
Supply
R
R
OUTPUT
R
A
A
A
* Compensation element
Do Not Ground!
Analyzer
V
TUO
1R2R
V8.1k27k33
V5.2k33k33
V3.3k02k33
3R
k02
k04
k0
15
1C
0.001µF
0.001µF
0.001µF
Test Circuit 1. Compensation (Bode Plot) Circuit
IN
1k R1
R2
33k
R3 C1
GND FB
SNKOUT
RETURN
MIC4043
Supply
* Compensation components
Test Circuit 2. Transient Response Circuit
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MIC4043 Micrel
MIC4043 4 November 2000
Transient Response 1a.
1.8V Output
V
IN
V
REG
1.8V
AC
Coupled
V
OUT
5.0V
AC
Coupled
6.0V
Overshoot 25mV
Turn On Transient Response 1b.
1.8V Output
V
IN
V
REG
1.8V
V
OUT
Transient Response 2a.
1.8V Output
V
IN
V
REG
2.5V
AC
Coupled
V
OUT
5.0V
AC
Coupled
6.0V
Overshoot 56mV
Turn On Transient Response 2b.
1.8V Output
V
IN
V
REG
2.5V
V
OUT
Transient Response 2a.
1.8V Output
V
IN
V
REG
3.3V
AC
Coupled
V
OUT
5.0V
AC
Coupled
6.0V
Overshoot 64mV
Turn On Transient Response 3b.
1.8V Output
V
IN
V
REG
3.3V
V
OUT
1 245V
November 2000 5 MIC4043
MIC4043 Micrel
Functional Diagram
SNK
FB
IN
V
REF
1.245V
GND
MIC4043
Functional Description
The MIC4043 combines a Gm amplifier, precision 1.245V
reference, and a pass transistor in a single package.
The operation of the MIC4043 is similar to conventional shunt
regulators such as the industry standard ’431. In a closed
loop system, the MIC4043 maintains the desired feedback
voltage at the FB pin by sinking current onto the SNK pin
proportional to the error voltage at the FB pin. The ratio of sink
current to error voltage is the transconductance of the device.
Reference
The MIC4043 uses a
high-side reference
. External voltage
dividers providing feedback to the MIC4043 will be
inverted
when compared to those used with
’
431-equivalent devices.
Behavior
The external feedback voltage is compared to the internal
high-side 1.245V reference.
If the feedback voltage, VFB, is less than VIN – VREF, the
amplifier provides no drive to the sink transistor. If the
feedback voltages is greater than VIN – VREF, the amplifier
drives the pass transistor which sinks current to ground.
PT
E
MIC4043 Micrel
MIC4043 6 November 2000
Applications Information
Replacement of ’431-Type Devices
Since the MIC4043 uses a high-side reference,
external
voltage dividers providing the feedback voltage will be
inverted when compared to those used with ’431-equiva-
lent devices.
The industry-standard ’431 is also typically used in series with
an opto-isolator LED. This configuration has a voltage drop of
at least 2.5V for the ’431 plus 1.4V for the LED (3.9V). More
recent lower-voltage shunt regulators require at least 1.25V
of headroom in addition to the 1.4V for the opto isolator, for
a total of 2.65V.
The MIC4043 removes the need to place the shunt reference
in parallel with the opto-isolator. The MIC4043 combines a
1.245V reference in conjunction with an error amplifier that
drives an NPN output transistor. The NPN transistor is
connected in series with the opto-isolator and regulates the
drive current in the opto-isolator. Unlike conventional shunt
regulators, the MIC4043 does not have to connect the shunt
reference in series with the opto-isolator. Only the NPN
output stage is in series with the opto-isolator, so the voltage
drop is just the saturation voltage or one transistor, typucally
160mV at full load
Compensation
The noninverting side of the error amplifier is connected to the
high-side reference; the reference is connected to the IN pin.
The inverting side of the error amplifier is brought out to the
FB pin. For some applications, no compensation is needed,
but for most, some resistor capacitor network is necessary
between the FB pin and GND pin. The value of the feedback
capacitance is application specific, but for most applications
100pF to 3000pF is all that is needed. Changing the feedback
capacitor changes the loop response; that is, phase and gain
margin. An empirical way to check overall system loop
response, if a network analyzer is not available, is to step load
the output of the systems from 10% to 100% of nominal load.
The resultant small signal response at the output of the
systems will provide an idea of which direction to go based on
the overshoot and settling time of VOUT.
Voltage Detector
IN
R1
Logic
Output
RPULL-UP
R2
33k
GND FB
SNK
VOUT
(FROM
POWER
SUPPLY) MIC4043
DISABLED
ENABLED
V 1.245V R2
R1 1
TRIP
=+
Figure 1. Voltage Detector
Figure 1 shows a simple voltage threshold detector with a
logic output.
High-Current Regulator
IN
40Ω1000pF
Q1
R
BIAS
I
BIAS
≥500µA
R2
33k
GND FB
SNK
MIC4043
V
OUT
= 2.5VV
IN
V 1.245V R2
R1 1
R1 33k, R2 33k
OUT
=+
∴= =
Figure 2. High-Current Regulator
For the high-current regulator shown in Figure 2, headroom
is equal to the saturation voltage of Q1 plus the saturation
voltage of the MIC4043 (VSAT(min) = 200mV).
w.‘ (‘nuHr
November 2000 7 MIC4043
MIC4043 Micrel
Off-Line 1.8V/2A Power Supply
R13
10Ω 1%
C11
1200µF
10V
R14
200Ω
1%
VOUT
+1.8V/2A
D4
12CTQ045 L2
5µH
C12
220µF
10V
U2b
2501
U3
249Ω1000pF
MIC4043
R10
72k
1%
R11
33k
1%
U1
MIC38HC43BN
VDD
VREF
GND
COMP
VOUT
RT/CT
FB
ISNS
2
1
4
3
8
5
7
6
R2
332k
1%
R3
332k
1% D1
1N4448
R4
34Ω
1%
C5
0.1µF
50/
63V
D2
18V
C7
470pF
63V
C6
470pF
63V
Q1
IRFIBE30G
L1
R5
1.21k 1%
R6
1.21k
1%
BR1
DBR1
C4
47µF
400V
U2a
2501
F1
85 to 264Vac
50/60Hz
Hot
Neutral
Return
T1
1
2
7
6
4
3
1A
Ground
R8
1.9Ω
1/4W
1%
R9
470Ω
1/2W
D3
UF4005
C9
100pF
1kV
R1
1k 1%
R7
14k 1%
C3
2200pF
400V
C2
2200pF
400V
C1
0.1µF
250V
C10
0.1µF
50/63V 80T 3T
20mH
C8
22µF
25V
10T
FBGND
SNKIN
V 1.245V R2
R1 1
TRIP
=+
Figure 3. Off-Line 1.8V/2A Power Supply
Figure 3a. 1.8V/1A Bode Plot
(θ margin = 102°)
Figure 3b. 1.8V/2A Bode Plot
(θ margin = 87°)
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MIC4043 Micrel
MIC4043 8 November 2000
Off-Line 2.5V/2A Power Supply
R13
10Ω 1%
C11
1200µF
10V
R14
200Ω
1%
VOUT
+2.5V/2A
D4
12CTQ045 L2
5µH
C12
220µF
10V
U2b
2501
U3
249Ω1000pF
MIC4043
R10
72k
1%
R11
33k
1%
U1
MIC38HC43BN
VDD
VREF
GND
COMP
VOUT
RT/CT
FB
ISNS
2
1
4
3
8
5
7
6
R2
332k
1%
R3
332k
1% D1
1N4448
R4
34Ω
1%
C5
0.1µF
50/
63V
D2
18V
C7
470pF
63V
C6
470pF
63V
Q1
IRFIBE30G
L1
R5
1.21k 1%
R6
1.21k
1%
BR1
DBR1
C4
47µF
400V
U2a
2501
F1
85 to 264Vac
50/60Hz
Hot
Neutral
Return
T1
1
2
7
6
4
3
1A
Ground
R8
1.9Ω
1/4W
1%
R9
470Ω
1/2W
D3
UF4005
C9
100pF
1kV
R1
1k 1%
R7
14k 1%
C3
2200pF
400V
C2
2200pF
400V
C1
0.1µF
250V
C10
0.1µF
50/63V 80T 3T
20mH
C8
22µF
25V
10T
FBGND
SNKIN
V 1.245V R2
R1 1
TRIP
=+
Figure 4. Off-Line 2.5V/2A Power Supply
Figure 4a. 2.5V/1A Bode Plot
(θ margin = 83°)Figure 4b. 2.5V/2A Bode Plot
(θ margin = 83°)
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November 2000 9 MIC4043
MIC4043 Micrel
Off-Line 3.3V/2A Power Supply
R13
10Ω 1%
C11
1200µF
10V
R14
200Ω
1%
VOUT
+3.3V/2A
D4
12CTQ045 L2
5µH
C12
220µF
10V
U2b
2501
U3
249Ω1000pF
MIC4043
R10
72k
1%
R11
33k
1%
U1
MIC38HC43BN
VDD
VREF
GND
COMP
VOUT
RT/CT
FB
ISNS
2
1
4
3
8
5
7
6
R2
332k
1%
R3
332k
1% D1
1N4448
R4
34Ω
1%
C5
0.1µF
50/
63V
D2
18V
C7
470pF
63V
C6
470pF
63V
Q1
IRFIBE30G
L1
R5
1.21k 1%
R6
1.21k
1%
BR1
DBR1
C4
47µF
400V
U2a
2501
F1
85 to 264Vac
50/60Hz
Hot
Neutral
Return
T1
1
2
7
6
4
3
1A
Ground
R8
1.9Ω
1/4W
1%
R9
470Ω
1/2W
D3
UF4005
C9
100pF
1kV
R1
1k 1%
R7
14k 1%
C3
2200pF
400V
C2
2200pF
400V
C1
0.1µF
250V
C10
0.1µF
50/63V 80T 3T
20mH
C8
22µF
25V
10T
FBGND
SNKIN
V 1.245V R2
R1 1
TRIP
=+
Figure 5. Off-Line 3.3V/2A Power Supply
Figure 5a. 3.3V/1A Bode Plot
(θ margin = 82°)
Figure 5b. 3.3V/2A Bode Plot
(θ margin = 80°)
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MIC4043 Micrel
MIC4043 10 November 2000
Off-Line 5V/2A Power Supply
R13
10Ω 1%
C11
1200µF
10V
R14
200Ω
1%
VOUT
+5.0V/2A
D4
12CTQ045 L2
5µH
C12
220µF
10V
U2b
2501
U3
249Ω1000pF
MIC4043
R10
72k
1%
R11
33k
1%
U1
MIC38HC43BN
VDD
VREF
GND
COMP
VOUT
RT/CT
FB
ISNS
2
1
4
3
8
5
7
6
R2
332k
1%
R3
332k
1% D1
1N4448
R4
34Ω
1%
C5
0.1µF
50/
63V
D2
18V
C7
470pF
63V
C6
470pF
63V
Q1
IRFIBE30G
L1
R5
1.21k 1%
R6
1.21k
1%
BR1
DBR1
C4
47µF
400V
U2a
2501
F1
85 to 264Vac
50/60Hz
Hot
Neutral
Return
T1
1
2
7
6
4
3
1A
Ground
R8
1.9Ω
1/4W
1%
R9
470Ω
1/2W
D3
UF4005
C9
100pF
1kV
R1
1k 1%
R7
14k 1%
C3
2200pF
400V
C2
2200pF
400V
C1
0.1µF
250V
C10
0.1µF
50/63V 80T 3T
20mH
C8
22µF
25V
10T
FBGND
SNKIN
V 1.245V R2
R1 1
TRIP
=+
Figure 6. Off-Line 5V/2A Power Supply
Figure 6b. 5V/2A Output Bode Plot
(θ margin = 61°)
Figure 6a. 5V/1A Output Bode Plot
(θ margin = 67°)
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{74Pj
A FJLi
November 2000 11 MIC4043
MIC4043 Micrel
Package Information
0.150 (0.0059)
0.089 (0.0035)
8°
0°
0.400 (0.016) TYP 3 PLACES
2.50 (0.098)
2.10 (0.083)
1.40 (0.055)
1.20 (0.047)
0.950 (0.0374) TYP
3.05 (0.120)
2.67 (0.105)
0.800 (0.031) TYP
1.12 (0.044)
0.81 (0.032)
0.10 (0.004)
0.013 (0.0005)
DIMENSIONS:
MM (INCH)
0.41 (0.016)
0.13 (0.005)
C
L
C
L
SOT-143 (M4)
MIC4043 Micrel
MIC4043 12 November 2000
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
© 2000 Micrel Incorporated
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